The Boston Red Sox brought back their World Series MVP a few weeks ago, but the most valuable retention of their off-season unfolded Thursday.

Nathan Eovaldi was the guy Boston could not afford to lose, and the club acted quickly to ensure his 100-mph fastball isn’t going anywhere.

The Red Sox signed Eovaldi to a four-year deal Thursday and will re-introduce him Monday from baseball's winter meetings in Las Vegas. Terms were not available, though MLB.com reported that the deal was for four years and $67.5 million.

In locking up Eovaldi, the Red Sox protected themselves against some significant churn in their pitching staff.

That’s a lot of firepower to potentially kiss off over two winters. So in a world where velocity wins, bringing Eovaldi back was crucial.

He showed just how valuable he could be down the stretch and in the postseason, serving as a starter, a late-innings fireman and, in an epic Game 3 of the World Series, a staff-saving immovable force.

Eovaldi held the Los Angeles Dodgers scoreless over six innings in relief, simultaneously keeping the Red Sox in the game while taxing their opponent’s bullpen. He eventually gave up an 18-inning, walk-off home run to Max Muncy, but Eovaldi’s outing might have been the single most important one of the Series.

“Honestly, I felt pretty privileged to watch what Nathan did tonight,” Porcello said after Game 3. “That was the most incredible (effort) I think I’ve seen.”

Eovaldi maintained his 100-mph heat through six-plus innings that night, perhaps the best illustration yet that his second Tommy John surgery – performed after the 2016 season – was a rousing success.

The Tampa Bay Rays signed him to a two-year, incentive-laden deal to come back by 2018 and then flipped him to the Red Sox in July.

The relatively low cost the Red Sox paid Tampa Bay – lefty Jalen Beeks - looks all the better now that Eovaldi will be in the fold four more years.

He posted a career-best 1.12 WHIP last season, with a Fielding Independent Pitching mark of 3.60 that closely mirrored his 3.81 ERA.

Eovaldi truly shined in the postseason, when he posted a 1.61 ERA over 22 1/3 innings. His three relief outings – including the Red Sox’s victorious Games 1 and 2 of the World Series – provided significant bridges to three of their most significant wins.

After Eovaldi’s Game 3 relief work, Boston’s hitters blitzed a taxed Dodgers bullpen, with Steve Pearce homering in Games 4-5 to earn Series MVP honors; he agreed last month to a one-year deal.

In 2019, Eovaldi should fold right in behind Sale, Porcello and David Price in Boston’s rotation. Come playoff time, should the Red Sox be in position to defend their title, his role once again may best be described as “power arm.”

He showed in October that he can take the ball anytime, no matter the stakes, and dominate.